1. Field of the Invention
The present invention generally relates to gas turbine engines and, more particularly, optimization of film cooling configurations therefor.
2. Description of the Related Art
Gas turbines, also called combustion turbines, are a type of internal combustion engine. They have an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to a gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature and pressure of the exhaust gas. Energy can be extracted in the form of shaft power, compressed air or thrust or any combination of these and used to power aircraft, trains, ships, generators, or even tanks.
Contemporary and next generation gas turbine engines generally demand that high pressure turbine (“HPT”) inlet temperatures be ever-increasing to allow for higher thrust-to-weight ratio and thermodynamic efficiency. This requires that durability measures for turbine components continue to meet the challenge and keep pace with the trend of increasing hot gas temperatures in the engine. For decades, this has been done with high-temperance materials, thermal barrier coatings, and cooler air flow that is routed from the upstream compressor through the inside passages of the component. The last of these durability measures, internal convective cooling, becomes film cooling when the coolant is leaked through small holes in an airfoil wall and out onto an external surface. Film cooling has afforded the greatest leaps in gas turbine engine performance and durability, allowing engines to operate at temperatures beyond the material limits of their components. Active external cooling is required to achieve adequate part life for these configurations. Thus, cooling may be applied to high temperature components at a penalty to the efficiency of the engine.
The purpose of film cooling in any turbomachinery is to protect the material surface of components at the location of cooling injection as well as downstream. Film cooling works by using a fraction of the ambient flow going into the engine from a bypass fan near the compressor and routing it internally through the inside of the HPT components, allowing it to escape through small cooling holes in the surface where it interacts with the mainstream hot engine flow. As the main flow mixes with the cooler injected flow, a film is created over the material surface. The introduction of a secondary fluid into the boundary layer at a temperature lower than the mainstream results in a reduction of the material surface temperature in the region downstream of the injection. In the right pressure gradients, the additional cooling mass flow also provides a thicker boundary layer and greater insulation from potentially-damaging hot flow. Generally, a two-dimensional array of discrete cooling holes on the component surface allows for a coalescence of film cooling coverage over the component. While slots have shown to have the greatest cooling effectiveness downstream from the injection location, rows of discrete cooling holes provide the necessary structural integrity for the increased thermal stress environment of a HPT in addition to the beneficial additive nature of cooling effectiveness due to repeating rows, relative to single or sparsely-spaced holes.
Continual advances in computing power have afforded greater capabilities in computational fluid dynamic (CFD) simulations for a turbomachinery designer. But even with these advances reliable simulation tools, physical models, and all-encompassing turbomachine design methods that consider individual 3-D geometries, pertinent flow conditions, and whole film cooling arrays in the process are not fully developed or available. Currently, design practices related to film cooling configurations involve a high amount of empiricism and may be missing important physical behaviors that have direct impacts on durability, especially for the hottest part that sees the harshest environment, the HPT. Industry proprietary methods tend to use non-dimensional dated film correlations that assume analytical cooling performance at one span (usually mid span) will be the same at all spans. The 3-D nature of HPT component surfaces and local effects of the film cooling array are not factored into durability designs for individual parts. This prohibits accurate knowledge of thermally-driven stresses for a specific part given its unique geometry. Unfortunately, due to the intense compute power required, CFD codes are typically not used in the process of film cooling design. While industry has taken advantage of contemporary computing power to design for optimal aerodynamic shapes and structural design, the same cannot be said for film cooling design. Accordingly, there is a need in the art for a design methodology including 3-D CFD simulations for improving film cooling of turbomachinery components.